Electroless copper plating on a variety of substrates has been used, for example, in the manufacture of printed circuit boards. The baths conventionally contain a soluble copper salt, a copper complexing or chelating agent, a reducing agent and stabilizer and brightener additives. Early baths using sodium hypophosphite reducing agent are quite stable but have very low plating rates. Baths using formaldehyde reducing agent, now widely used, have faster plating rates but less stability than the hypophosphite baths. It is desirable to avoid formaldehyde baths because of the toxic hazard of formaldehyde in the workplace. Cyanides have frequently been used as complexing agents and they also present a toxic hazard and disposal difficulties.
Electroless copper plating tends to be self-limiting as plating stops, or is drastically slowed, as a significant thickness of plate is deposited. Conventionally in the manufacture of printed circuit boards, copper is electroplated over a thin strike of electroless copper. There is a need for an electroless copper plating bath of sufficient stability and turnover life that plates rapidly enough to plate the entire thickness of copper on the printed circuit boards, suitably to a thickness of 1 mil or more.
Dimethylamine borane has also been used as reducing agent in electroless copper plating baths. Pearlstein and Wightman, U.S. Pat. No. 3,370,526 and Plating, Vol. 60, No. 5, pp. 474-6, May 1973, deposited copper strikes from a bath containing copper sulfate, EDTA disodium salt, DMAB, and ammonium hydroxide. Arisato and Korijama, U.S. Pat. No. 4,138,267 disclosed baths with borane reducing agents, hydroxyl substituted ethylenediamine copper-complexing agents, adjusted to 12 to pH14 with alkali hydroxide, and containing cyanide or ferrocyanide stabilizers. Dimethylamine borane has also been used in acid or neutral electroless copper plating baths: U.S. Pat. No. 4,143,186 and U.S. Pat. No. 3,431,120.